With the rapid progress of digital technology and the development of semiconductor manufacturing process, the electronic industry has developed highly integrated and powerful processor or graphic chips. However, these powerful digital chips can only operate with the digital input signal, while most of the electrical signals are analog. Therefore, many analog-to-digital converters (ADC) have been developed to meet different demands, such as, high speed or high resolution ADC. The analog electrical signal is usually generated by sensors, such as, voltage sensor, luminance sensor, temperature sensor, ultrasonic sensor, speed sensor or humidity sensor. In particular, the rapid development of sensors applied to Microelectro-mechanical System (MEMS) in recent years has gained popularity in many consumer electronic products. For example, Wii from Nintendo uses a MEMS-based three-axial acceleration sensor to work with wireless controller to achieve the highly creative entertainment. In addition, touch panel is another popular application.
These applications use sensors and amplifier to connect to ADC. Among them, Σ-Δ (sigma-delta) ADC is a common choice of ADC.
FIG. 1 shows a schematic view of a functional diagram of the conventional apparatus for converting inductive capacitance. As shown in FIG. 1, an apparatus 1 for converting inductive capacitance includes a sensor 10, a sensor amplifier 20, a bias circuit 30 and ADC 40, where sensor amplifier 20 amplifies the output signal from sensor 10, and ADC 40 converts into digital signals. Bias circuitry 30 provides suitable bias voltage for sensor amplifier 20 and ADC 40.
FIG. 2 shows a detailed view of FIG. 1. As shown in FIG. 2, the electric model of sensor 10 shows a capacitor CS and equivalent input impedance R. Capacitor CS has a capacitance change ACS caused by the external environmental change. Under the condition of bias voltage Vbias, capacitor CS voltage change is ΔVCS, which is amplified by sensor amplifier 20 and input to ADC 40. Take a one-stage Σ-Δ ADC as an example. ADC 40 includes a first-stage converter circuit 41 and a comparator 45, where first-stage converter circuit 41 further includes a subtracter 42, an adder 43, a delay relay 44 and a digital-to-analog converter (DAC) 46. DAC 46 converts the digital output signal Vout from comparator 45 into analog signal. Subtracter 42 finds the difference between the output signal of sensor amplifier 20 and the output signal of DAC 46. Adder 43 adds the output signal of delay relay 44 to the difference, and outputs to delay relay 44 so as to complete the entire ADC operation. As Σ-Δ ADC is a commonly known technique, the above description is only to highlight the key points.
In FIG. 2, stray capacitor C2 is connected to capacitor CS and ground. Stray capacitor is an additional equivalent capacitor generated by errors in manufacturing process or circuit layout, and the capacitance of capacitor C2 will vary with different manufacturing process and circuit.
In addition, in a conventional Σ-Δ ADC structure, to improve the resolution of ADC, a structure with a plurality of serial stages is usually used. That is, the output signal of first-stage converter circuit 41 can be passed to the next stage converter circuit, and the last stage converter circuit is connected to the comparator.
However, the conventional technique has the drawback of requiring a bias circuit able to generate a bias voltage and a first-stage amplifier so as to increase the sensing sensitivity. However, it is a difficult challenge for the general IC fabrication process to overcome the noise in the bias circuit, and also difficult to integrate into the other existing function blocks operating at low voltage.
Another drawback of the conventional technique is requiring a high quality amplifier to amplify the low inductive voltage to the voltage range processable by ADC. As the amplifier requires a large size chip area, the chip cost increases and the offset, gain and noise of the amplifier will also increase the signal error.
Yet another drawback of the convention technique is the accuracy of the overall ADC by the stray capacitor due to manufacturing errors or circuit layout, which also varies with the manufacturing process and circuit, leading to the unstable ADC.
Hence, it is imperative to devise an apparatus able to directly convert the capacitance to digital signal, by using ADC to directly convert the low level output signal to digital signal to save the sensor amplifier and the bias circuit to facilitate a smaller-size chip area, as well as eliminating the unstable problem of ADC caused by stray capacitor and increasing the ADC accuracy.